1. Field of the Invention
This application is a continuation in part of U.S. Ser. No. 398,734 filed July 15, 1982 and now abandoned. The present invention is directed to a strip cast aluminum alloy suitable for use in the manufacture of deep drawn and wall-ironed articles such as cans and the like.
2. The Prior Art
In recent years, aluminum alloys such as the Aluminum Association specification 3004 have been successfully fabricated into two piece beverage cans by deep drawing and ironing. The expanding use of the two piece aluminum cans has created a need for aluminum alloy sheet for forming the can body that not only possesses the required combination of formability and strength properties but is also economical to manufacture.
Typically the aluminum alloy 3004 sheet useful in the production of deep drawn and ironed beverage cans is cast by direct chill casting an ingot having a thickness of about 20-25 inches. The ingot is homogenized at 950.degree.-1125.degree. F. for 4-24 hours and then subjected to hot rolling wherein the ingot is passed through a series of breakdown rolls maintained at a temperature of 400.degree.-900.degree. F. to reduce the ingot in thickness to a reroll gauge of about 0.130 inch.
Thereafter the reroll stock is subjected to an annealing step wherein the stock is heated at 600.degree.-900.degree. F. for 0.5-3.0 hours to effect recrystallization of the metal structure. The annealed reroll stock is subjected to a final work hardening step wherein the reroll stock is cold rolled (room temperature rolling) to a final gauge of about 0.013 inch or about 90% of its original thickness to produce the substantially full hard (H19) temper required for two-piece can body stock.
In spite of the successful use in can-making of direct chill ingot cast aluminum alloy 3004, economic and energy considerations would favor the manufacture of the aluminum sheet by continuous strip casting route. In this process, the molten aluminum is cast and solidified into a thin web of one inch or less in thickness so that subsequent rolling is reduced to a minimum and the costly step of hot rolling is eliminated.
In the manufacture of continuous strip cast aluminum alloy 3004 for can manufacture, the thin, e.g., 0.2-1.0 inch, solidified cast web is typically reduced in thickness to a gauge of about 0.008 to 0.017 inch and generally about 0.013 inch by cold rolling with an intermediate recrystallization anneal at about 600.degree.-900.degree. F. Thereafter, as in the manufacture of direct chill ingot cast stock, the thinned, annealed stock is subjected to a final work hardening step by cold rolling to a final gauge of about 0.013 inch to produce the H19 temper required for can body manufacture.
Although the continuous strip cast aluminum alloy 3004 is advantageously utilized for many fabricated products, such stock has not been used extensively for the manufacture of drawn and wall-ironed can bodies.
In the production of two-piece drawn and wall-ironed beverage cans, circular discs or blanks are cut or punched from the cold worked sheet for deep drawing into the desired shape. The blank is first drawn to approximately the final diameter cup. The sidewalls are then reduced in thickness in one or more wall-ironing operations.
Because of the nature of the working stresses incurred during wall-ironing of the deep drawn shell, when continuous strip cast aluminum alloy 3004 is subjected to wall-ironing, deep grooves may appear on the finish can which is referred to in the art as "galling". Galling adversely affects the acceptability of the can product and the effectiveness of the can manufacturing process. Galling is not normally observed during wall-ironing aluminum sheets of the same alloy composition produced from direct chill ingot casting.
A second problem encountered in the manufacture of deep drawn and wall-ironed containers from strip cast aluminum alloy is the problem of "earing". Earing manifests itself as a scalloped appearance around the top edge of the cup during the deep drawing cup formation step of the draw and wall-iron processing of the aluminum sheet.
The scallops, or ears, represent an almost universally undesirable feature of the cup as the ears must be removed in order to present a smooth or flat upper lip on the cup. This of course necessitates cup trimming prior or subsequent to wall-ironing, with an attendant increase in production costs and material waste.
The level of earing in a drawn cup is determined by the following equation: ##EQU1## where he is the distance between the bottom of the cup and the peak of the ear and ht is the distance between the bottom of the cup and the valley of the ear.
To be acceptable for can making, the aluminum alloy sheet when processed into a cup must exhibit a level of earing of no more than about 3.5% and preferably less than about 3% earing. The level of earing experienced with commercially available continuously cast strip of aluminum alloy 3004 is generally in the range of 5% or more.
The art has addressed the problems encountered in the manufacture of deep drawn and wall-ironed cans from strip cast aluminum alloy without apparent success. Representative of this art is U.S. Pat. No. 3,834,900 assigned to the American Can Company which discloses a special strip cast, high strength aluminum alloy suitable for can making applications, e.g., can body or ends wherein the alloy is composed of about 2.8% magnesium, 0.65% iron and 0.25% each of silicon, copper and manganese, the balance being aluminum and incidental impurities. In a later patent to the same assignee, U.S. Pat. No. 4,111,721, the galling problem encountered with continuous strip cast aluminum sheet in the manufacture of container bodies using aluminum alloy 3004 is alleviated by heat soaking the cast strip at 900.degree.-1150.degree. F. for 16-24 hours. U.S. Pat. No. 3,930,895 discloses strip cast aluminum alloy 3004 suitable for the manufacture of can bodies which is modified ot have a high (1.6-3.0) manganese content. Sheet metal stock formed from a strip cast web of the modified alloy is homogenized at 950.degree.-1050.degree. F. to impart a gall-free character to the alloy sheet.
In U.S. application Ser. No. 398,735 entitled "Process for the Manufacture of Continuous Strip Cast Aluminum Alloy Suitable for Can Making", filed July 15, 1982, there is disclosed a process for the preparation of non-galling, low earing can stock from continuously cast aluminum strop suitable for deep drawing and wall-ironing into beverage containers wherein the molten aluminum material is cast by continuous strip casting into a web generally of an inch or less in thickness. The strip material is heated to a temperature of from 950.degree. to 1150.degree. F. for a time sufficient to homogenize the alloy, such time being generally about 10 to about 25 hours. The homogenized strip material is cold rolled to effect a first reduction in sheet thickness of at least 25%, and generally about 50 to 85%, heated to a recovery temperature of about 400.degree. to 475.degree. F. for 2 to 4 hours, and then subjected to a second cold rolling to effect a reduction in thickness of at least 10%, and generally 10 to 50%. The cold rolled sheet product is thereafter heated to effect recrystallization of the grain structure and then subjected to effect a final reduction in thickness of at least 50% of the original thickness of the sheet to impart a hard temper to the sheet.
To effect the most advantageous reduction in earing, the sheet is subjected to a second recovery heating of about 450.degree. to about 525.degree. F. for 0.5 to 3 hours intermediate between the second cold reduction and the recrystallization heating step.
Although the process of the copending application provides continuous strip cast aluminum alloy 3004 exhibiting non-galling and low earing characteristics, the alloy sheet when fabricated into a two piece drawn and wall-ironed can exhibits a marginal level of buckle strength, that is, the ability of the can to withstand high internal pressure without bottom inversion.
Buckle strength is determined by applying pressure within a drawn and wall-ironed can and then gradually increasing the pressure until the bottom end of the can deforms and bulges out, i.e., it buckles. The pressure at which the bottom buckles is then designated as the buckle strength. To be acceptable as can body stock a can formed from the alloy sheet must exhibit a buckle strength of at least 90 pounds per square inch (psi), and preferably between 95 and 100 psi. Cans drawn and wall-ironed from a hard temper sheet of the continuous strip cast aluminum alloy 3004 homogenized at 1050.degree.-1100.degree. F. to eliminate galling exhibit a buckle strength of about 85 psi. When the cold roll/recovery-recrystallization heating steps of the copending application are employed to process the homogenized alloy 3004 to low earing can stock, the buckle strength of cans formed from the stock ranges from 85 to 90 psi.